Temperature and thermal conductivity measurements are frequently made in connection with various chemical and physical investigations, in engineering applications as in the optimization of processing parameters, and in determining the properties of various materials. Ordinarily, conventional devices provide perfectly satisfactory measurements of temperature and thermal conductivity, particularly where the bulk size of the sample being measured is large compared to the measuring device and rapid response time is not required. Typical conventional temperature measuring devices are thermometers, conventional thermocouples, pyrometers and thermistors.
In many applications, conventional temperature measuring devices do not provide satisfactory results. Such situations arise where the sample is extremely thin or where the region of interest for measuring the temperature is highly confined (e.g., a thin polymer layer). Also, in many applications rapid response time is required. Such conditions arise in a number of well known processes. For example, in xerography, images are typically fused by passing a toned paper through a pair of heated rollers. Another example is the coating of surfaces with polymer using a melt extrusion process. These processes require accurate determination and control of temperatures in a small, confined region such as a material-substrate interface. In addition, in many applications, temperature measurements are needed within a flexible, electrically insulating, thin layer. Under such circumstances the temperature measuring device should withstand extensive bending without adverse effects. For purposes of this application, electrically insulating layers should typically have an electrical resistivity greater than one ohm cm.
Conventional temperature probes are often too bulky to fit into the region where temperature is to be measured. Moreover, their heat capacities and thermal conductivities are typically so large as to effect measurements on extremely thin samples or where the region of interest is extremely thin. Also, the large mass of conventional probes makes their response times too slow for many applications. Optical techniques including standard pyrometers may also be used to measure temperatures, but these techniques require a direct optical path to the region being measured. Often, such a direct optical path is not available because of the equipment associated with the process of interest.
A number of references have disclosed thin film thermocouples. For example, D. L. Decker et al. in an article entitled "Thermal Properties of Optical Thin Film Materials", NBS Special Publication 727, Laser Induced Damage in Optical Materials 1984, Government Printing Office, D.C., 1986, pp. 291-297, reports the use of chromel/alumel thin film thermocouples in the measurement of thermal conductivity of thin films of aluminum oxide and silicon dioxide on rigid supports. These thermocouples were made by electron beam deposition and had a thickness of 1000 Angstroms.
U.S. Pat. No. 4,795,498 (D. Germanton et al.), issued Jan. 3, 1989, discloses thermocouples made of layers of metal on Mylar sheet which is laminated onto a rigid structure such as paper board. The layers of metal have a thickness between 2000 and 5000 Angstroms and are formed by vapor deposition or vacuum sputtering and requires laying down a layer of varnish between the deposition of the two metals. Bismuth and tin were used as the thermocouple metals. Use of alloys as the thermocouple metal was avoided because the metal deposition process used in the fabrication of the thermocouples alters the composition of the alloy. There is also disclosed a process for fabricating individual thermocouple units of very low cost used primarily as probes for human body temperature measurements.
U.S. Pat. No. 4,779,994 (T. E. Diller et al.), issued Oct. 25, 1988, discloses a heat flux gage employing various thin film layers as thermocouples. This reference exemplifies the prior art at the time of filing of the instant application in that the substrate is rigid so as to prevent breaking of the thin film thermocouple. This reference does not suggest in any way that a durable and reliable ultrathin film thermocouple could be made that was flexible. Similarly, Decker et al., in NBS Special Publication 727, Laser Induced Damage in Optical Materials: 1984 (Government Printing Office, Washington, D.C., 1986), pp. 291-297, describes certain thermal conductivity experiments in which an ultrathin thermocouple is deposited on a sapphire substrate. Again, this reference discloses a rigid thin film temperature measuring device and not a flexible device.
Other references provide valuable background for an understanding of the invention. For example, U.S. Pat. No. 3,305,393 (D. T. Breckenridge), issued Feb. 21, 1967, discloses a method of making a thermopile made up of a plurality of thermocouples connected in series by use of a grooved substrate. Bismuth and nichrome were used as thermocouple metals and these metals were deposited by an evaporation technique.
U.S. Pat. No. 4,091,138 (T. Takagi et al.), issued May 23, 1978), discloses a method of depositing a metal on an insulating surface using a cluster ion plating method. The method is supposed to produce a dense, electrically conductive metallic coating with good adherence without use of an adhesive.
U.S. Pat. No. 4,229,476 (H. Forster et al.), issued Oct. 21, 1980, describes certain thin film electrical circuit components made by using various oxides to provide adherence of the metal film to an insulating substrate such as paper or synthetic resin. U.S. Pat. No 4,720,401 (P.S.C. Ho et al.), issued Jan. 19, 1988, describes a procedure for increasing the adhesion of metals to organic substrates by heating the substrate to a temperature range of about (0.6-0.8) T.sub.c where T.sub.c is the curing temperature of the substrate.
Thin film thermocouples are also disclosed in U.S. Pat. No. 4,963,195 (S. Kodato et al.), issued Oct. 16, 1990. Here, films of silicon-germanium alloy serve as one of the thermocouple leads and a metal conductor as the second thermocouple lead. These thermocouples are used as part of a power detector. Such detectors exhibit good linearity characteristics in the low power range. U.S. Pat. No. 4,969,956, (K. G. Kreider et al.) issued Nov. 13, 1990, describes a transparent thin film thermocouple made with indium tin oxide and indium oxide. These thermocouples are useful for measuring temperature of transparent materials without disturbing their optical properties. U.S. Pat. No. 5,033,866 (T. Kehl et al.), issued Jul. 23, 1991, discloses a multiple thermocouple sensor in which thick film thermocouples are mounted on a carrier plate. These devices are useful in various thermal analysis ovens.
An article entitled "Thin Film Thermocouples for Use in Scanning Electron Microscopy", by Clark et al., Proceeding of the Ninth Annual Scanning Microscope Symposium, ITT Research Institute, April 1976, pp 83-90, discloses extremely thin thermocouples for use in vacuum equipment and scanning electron microscopes. The application described in this article does not require a flexible temperature measuring device and the reference does not describe nor teach a flexible temperature measuring device as that term is usually used and as that term is used in this application. Indeed, the fabrication procedure described in the Clark et al. article shows that the authors believed, as other scientists believed at that time, that ultrathin film thermocouple devices could not be flexed without destroying the device. This is evident from the fact that nylon is first stretched over a rigid holder having a hole therethrough and then thermocouple metal layers are deposited on a portion of the nylon which covers the hole. Thereafter, the nylon is not stretched, flexed (bent) or deformed.
It is desirable to provide a temperature measuring device sufficiently small in size and thickness to measure temperature in confined spaces such as a surface interface and sufficiently robust to remain functional under a variety of conditions including use on a flexible layer, and to provide a temperature measuring device with sufficiently small heat capacity and thermal conductivity so that the measuring device does not affect the temperature measurements even with extremely small samples. It is also desirable to provide a temperature measuring device with sufficiently small mass so that response times are short and to provide a differential temperature measuring device useful for measuring thermal conductivity, phase transitions, etc., on a thin layer of insulating material, particularly a thin flexible layer of insulating material. Such a flexible temperature measuring device might be used in applications where the flexible layer is continuously flexed or bent so as to conform to a nonplanar or irregular surface. For example, often various kinds of processing are carried out on flexible elements such as sheets and the temperature of the flexible elements is an important parameter in the process.
In the art of xerography, the process of thermal assisted transfer is of considerable importance. Here, a thermoplastic bearing receiver sheet is wrapped around a heated roller and fed into a nip formed by the heated roller and a second roller which may or may not be heated. While in the nip, the receiver sheet contacts an image bearing member where the image is formed using certain toners. Toner transfer is accomplished under specified conditions of temperature and pressure. Immediately on exiting the nip, the receiver is separated from the image bearing member, with the image completely transferred to the receiver sheet. The temperature of the receiver sheet is critical to successful transfer of the image. Conventional temperature measuring techniques do not provide such accurate and reliable temperature measurements. The smallest conventional thermocouples are about 10 to 20 micrometers thick. This is much too thick to feed through a nip such as described above without disturbing the nip geometry and can cause damage to the nip material or apparatus. Also, the mass of the device would draw heat away from the receiver surface and result in inaccurate measurements. Other techniques such as infrared pyrometry and other noncontacting methods would not be effective in measuring the temperature within the nip because the geometry of the rollers would interfere with the measuring technique. Similar considerations apply to other processes such as toner fusing where the image is permanently fixed to the receiver (usually paper). Other areas where ultrathin thermocouples are useful are melt extrusion, heat transfer characteristics of polymer coatings, and characteristics of polymer coatings.
It is highly desirable to have a temperature measuring device that can measure the temperature of the flexible elements during processing directly so as to assure high efficiency of the process.